s390/kexec_file: Prepare setup.h for kexec_file_load

kexec_file_load needs to prepare the new kernels before they are loaded.
For that it has to know the offsets in head.S, e.g. to register the new
command line. Unfortunately there are no macros right now defining those
offsets. Define them now.

Signed-off-by: Philipp Rudo <prudo@linux.vnet.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>